In the fabrication of semiconductor devices, semi-conducting substrates or wafers of silicon nature are generally used as a foundation for building electronic devices thereon. To properly identify these substrates or wafers, identification marks must be suitably placed on the wafer such that the wafers can be readily identified throughout the fabrication process. The fabrication process for semiconductor devices may consist of as many as several hundred processing steps which are conducted at different processing stations by different fabrication equipment. A reliable method of identification of these wafers by their lot numbers is therefore very important not only from a production control point of view, but also for future tracking of the semiconductor devices that are built when quality issue or discrepancy arises.
Traditionally, identification marks are provided on a substrate or wafer on the patterned side where electronic devices are built. A typical wafer, which may have a size of 100 mm, 125 mm, 150 mm or 200 mm is shown in FIG. 1. The wafer 10 which has a wafer center 12 and a flat edge 14. The wafer 10 can be suitably marked in an area 16 generally defined at a location near the flat edge 14. In marking an identification number, a laser beam can be used to scribe directly the identification marks into a silicon layer, or first through an insulating layer which is frequently formed on a silicon wafer and then into the silicon layer.
FIG. 2 shows an enlarged, cross-sectional view of a scribe mark 20 formed on the silicon wafer 10 of FIG. 1. It is seen that on wafer 10, electronic device 22 is built on the patterned surface 24 of the wafer. The scribe mark 20 represents a volume that is defined by the width of the bottom 26 of the mark times the height of the side wall 28 times the length (not shown) of the mark made in the wafer surface 24. The volume of the scribe mark 20 represents an amount of silicon that is removed from the wafer surface 24 by a laser scribing machine or other suitable cutting tools. The removal of the volume of silicon material from the mark 20 in the laser scribing process creates silicon particles or dust as a potential source of wafer contamination. The silicon particles or dust may contaminate the wafer surface or the interior surface of the process chamber which may then fall off the chamber wall onto the wafer surface. Even though a wet chemical cleaning process is normally performed to clean out the silicon particles after a laser scribing process, it may not be sufficiently effective in eliminating all contaminating particles.
During a laser scribing process, a high energy laser beam bombards the wafer surface such that silicon particles from the bombarded area form clusters or bumps of silicon on the wafer surface near the mark it created. In the subsequent processing steps for the wafer, a wafer clamp or other wafer holding device may be used for fixing a wafer on a wafer stage in certain process machines, the bumps of silicon material may be crushed into silicon particles as additional sources of contamination for the wafer surface.
An additional problem associated with the method of laser scribing the patterned side of a wafer has been observed when a chemical mechanical polishing (CMP) is used for planarizing a wafer surface. When the wafer surface is planarized by the CMP process, a very smooth surface is obtained which essentially smears out the identification mark and makes it illegible under a light source. As a result, the identification of the lot number or any other marks on the patterned surface of the wafers becomes a very difficult task.
It is therefore an object of the present invention to provide a method for marking a wafer surface that does not have the drawbacks or shortcomings of the conventional wafer marking techniques.
It is another object of the present invention to provide a method for marking a wafer surface that minimizes the wafer contamination problem caused by silicon particles produced by a laser scribing process.
It is a further object of the present invention to provide a method for marking a wafer surface that is capable of producing a legible identification mark even after a planarization process conducted by a chemical mechanical polishing technique.
It is still another object of the present invention to provide a method for marking a wafer surface by marking the non-patterned or the backside of a wafer.
It is another further object of the present invention to provide a method for marking a silicon wafer that is compatible with a planarization process by first providing a mark on the non-patterned side of the wafer and then reproducing such mark on the patterned side of the wafer after such planarization process.
It is yet another object of the present invention to provide a method for marking a wafer surface by first forming an insulating layer on the surface of the non-patterned side of the wafer and then laser scribing an identification mark in the wafer surface.
It is still another further object of the present invention to provide a method for marking a wafer surface by first providing an oxide layer on top of a non-patterned wafer surface, scribing an identification mark through the oxide layer into the wafer surface and then removing the oxide layer.
It is yet another further object of the present invention to provide a wafer that has a recognizable identification mark on the pattern side of the wafer after the non-patterned side of the wafer is planarized by a grinding process.